1. Field of the Invention
The present invention relates generally to differentials and more particularly to limited-slip differentials.
2. Background Art
An axle with a wheel affixed to each end may be driven by a power source to propel a vehicle in a substantially straight line. However, when making either a left or right turn, the wheel on the outer end of the axle travels a greater distance than the wheel on the inner end of the axle, causing the outer wheel to rotate faster than the inner wheel. This generally leads to twisting of the axle, and often results in wheel hop and/or axle breakage.
Differentials having a single input member, which drive two output members in a manner that permit the speeds of each of the output members to differ have been known. Differentials emerged from the need to have both output wheels of a driven axle to rotate at different speeds. Typical current day differentials still remain quite similar to the first known successful differential, invented in 1827 by Onesiphore Pecqueur, a Frenchman. Today""s differentials have an input gear that meshes with a ring gear, as in the earlier differential by Pecqueur. The ring gear drives a casing that carries bevel toothed pinions, which mesh with left and right side gears that power the left and right wheels, respectively. Both wheels rotate at the same speed, when the wheels travel a straight line, causing the pinions to remain stationary in the rotating casing. When making a turn, the pinions allow the wheels to rotate at different speeds, by rotating at their respective axes in the rotatable casing.
Torque from the input gear is split into two substantially equal components, which are distributed substantially equally to the left and right wheels. Consequently, if one of the driven wheels rotates on ice or mud, the wheel on the ice or mud spins, while torque to the wheel not on the ice or mud is reduced. This condition increases the risk of a motor vehicle having such a differential of becoming immobilized on substantially slippery surfaces.
When a motor vehicle travels in a curvilinear direction, such as when the vehicle turns left or right, or travels in a direction other than a straight line, such as on dry pavement, a relatively small speed difference occurs between the left and right wheels. However, when the vehicle travels on ice, snow, or through mud, the differential speed between the left and right wheels increases substantially beyond the relatively small speed difference which occurs between the wheels on dry pavement. In this instance xe2x80x9cslip limitationxe2x80x9d is desirable otherwise the vehicle may become immobilized. On the other hand, slip limitation is not desirable in normal driving situations since slip limitation may adversely affect directional control of the vehicle and accelerate tire wear.
xe2x80x9cLimited slip differentialsxe2x80x9d have been known, which limit the speed differentiation or xe2x80x9cslip,xe2x80x9d so that some of the torque being delivered by the input gear is transferred from the wheel that slips to the wheel that has more traction, and, therefore, aids in moving the vehicle in the preferred direction.
These limited slip differentials (LSD) have been developed to overcome the above mentioned shortcomings of a conventional differential, and work on various principles, but generally are all intended to limit the speed difference or xe2x80x9cslipxe2x80x9d between the left and right driven wheels. Such limited slip differentials generally limit the speed differentiation or xe2x80x9cslipxe2x80x9d between the driven wheels, so that some of the torque being delivered by the input gear is transferred from the wheel that slips to the wheel that has more traction, and, therefore, aids in moving the vehicle in the preferred direction. However, most of these limited slip differentials do not limit the speed difference between the driven wheels adequately at a substantially large speed differential or slip between the driven wheels. Other limited slip differentials have valves, clutches, pistons or other special components that add to the complexity and cost of production.
There is a need for a limited slip differential, which limits the speed differentiation or xe2x80x9cslipxe2x80x9d between the driven wheels, and gradually increases torque to the non-slipping wheel, as the rotational speed difference between the slipping wheel and the non-slipping wheel increases. The rate of increase of the torque to the non-slipping wheel should increase, as the speed difference between the slipping wheel and the non-slipping wheel increases. The rate of increase of the torque applied to the non-slipping wheel should preferably increase at a greater rate than the rate of increase of the speed difference between the wheels. Torque transfer should be substantially zero or minimal between both driven wheels, when there is substantially no speed difference or slip between the driven wheels, since such torque transfer is not required when there is substantially no speed difference or slip between the driven wheels. The rate of increase of the torque applied to the non-slipping wheel should be adjustable by modifying physical characteristics of the limited slip differential. Such limited slip differentials should be inexpensive, durable, long lasting, easy to manufacture and install, either as an original equipment item or as a retrofit, easy to maintain, and require a minimum of maintenance.
When applied to a motor vehicle, the limited-slip differential should provide slip limitation or torque transfer substantially only when speed difference between the driven wheels of the motor vehicle is beyond normal driving limits. The torque transfer should increase as the speed difference between the driven wheels increases, and aid in decreasing the probability of a vehicle becoming immobilized during slippery conditions. The rate of increase of the torque applied to the non-slipping wheel should preferably increase at a greater rate than the rate of increase of the speed difference between the driven wheels. Such limited slip differentials should be inexpensive, durable, long lasting, sturdy, easy to manufacture and install, either as an original equipment item or as a retrofit, be capable of having a substantially similar form factor and size as original equipment items, be easy to maintain, and require a minimum of maintenance. The limited slip differential should also be compatible with anti-lock braking systems, two, four, and multiple drive vehicles, and be useable in vehicle and other applications.
Different limited slip differentials have heretofore been known. However, none of the limited slip differentials adequately satisfies these aforementioned needs.
Mechanical limited slip differentials have been disclosed. U.S. Pat. No. 4,516,443 (Hamano et al) and U.S. Pat. No. 4,939,953 (Yasui) disclose mechanical limited slip differentials, each having a set of clutches connected to a casing, which alternate with another set of clutches attached to one or two side gears. The clutches are kept in contact by a preloaded spring, through which both wheels are always contacted to some extent. These limited slip differentials xe2x80x9cbindxe2x80x9d the wheels together, starting with a specified amount of torque, but increasing the slip limitation when the input torque increases. Therefore, there is always a certain amount of torque transfer between the left and right wheel, even in normal driving when it is not needed.
Parallel-axis limited slip differentials have been disclosed. U.S. Pat. No. 5,244,440 (Ichiki et al) discloses a parallel-axis differential having a plurality of pinions, in which friction produced by meshing and rubbing of the pinions provides slip limitation. U.S. Pat. No. 5,302,159 (Dye et al) discloses a parallel-axis differential in which friction produced by end thrust of pinions and side gears to a casing, resulting from specific helical tooth angles, provides slip limitation.
U.S. Pat. No. 3,292,456 (Saari) and U.S. Pat. No. 3,738,192 (Belansky), and European Patent No. 130806A2 (Quaife) disclose parallel-axis differentials having pinions that are continuously meshed in a circumferential manner, which provide slip limitation, resulting from an increase in frictional surfaces at tooth meshing points and between pinions and casing. Slip limiting action of these aforementioned parallel-axis differentials are dependent on input torque and not on the speed difference between the driven wheels. Such limited slip differentials may be acceptable for driven wheels on dry pavement, but are not adequate on slippery surface.
Parallel-axis differentials using the resistance to pumping action have been disclosed. U.S. Pat. No. 3,251,244 (Nickell), U.S. Pat. No. 4,272,993 (Kopich), and U.S. Pat. No. 5,083,987 (Korner et al) disclose parallel-axis differentials having pinions in pairs, in which meshing teeth also function as pumps, and the resistance to pumping action provides slip limitation. U.S. Pat. No. 4,630,505 (Williamson) also discloses a parallel-axis differential, which also functions as a pump, in which the resistance to pumping action provides slip limitation, the side gears being internally toothed, instead of having external toothing. These differentials produce increasing slip limitation, as the speed difference between the driven wheels increases. However, although such low displacement designs result in low slip limiting capacity.
U.S. Pat. No. 5,232,410 (Yanai) and U.S. Pat. No. 5,162,024 (Yoshiba) disclose limited slip differentials that use pumping resistance for slip limitation. Piston pumps are used instead of gear pumps, as seen in the parallel-axis differentials described above. A round cylinder block carrying pistons in a radially outward direction is attached to a side gear. Another side gear has an internal surfaced cam that drives the pistons to pump fluid across a restricted outlet. Yanai also discloses a torque curve for different orifice sizes relative to differential speed. Although these differentials produce increasing slip limitation with increasing speed difference at the driven wheels, such devices are costly as a result pistons, cylinders, and spool valves required in the differential.
Vehicle drive train couplings using the resistance to pumping action have been disclosed. U.S. Pat. No. 3,869,013 (Pagdin et al) and U.S. Pat. No. 5,456,642 (Frost) disclose vehicle drive train couplings, using resistance to pumping action of gear pumps to transmit torque across the couplings. Torque curves in relation to the speed difference across the couplings are also disclosed.
Limited slip differentials having viscous couplings have been disclosed. U.S. Pat. No. 4,869,129 (Hazebrook) and U.S. Pat. No. 5,162,023 (Kwoka) disclose viscous couplings attached to a casing and an output member. The coupling units have stacks of alternating discs immersed in a viscous fluid medium. The presence of speed difference between left and right wheel causes the alternating discs to rotate at different speeds, shearing along the fluid medium between the stacks of the alternating discs. The shearing resistance of the viscous fluid provides slip limitation or torque transfer between the left and the right wheel. The fluid shearing resistance increases as the speed difference increases, thus resisting the xe2x80x9cslipxe2x80x9d between the two wheels with increasing force, as the speed difference between the two wheels increases; however, the maximum torque capacity of the viscous coupling is relatively small.
U.S. Pat. No. 4,012,968 (Kelbel) and U.S. Pat. No. 5,611,746 (Shaffer) disclose limited slip differentials that limit the slip above a predetermined differential speed. The devices have clutches that are engaged by fluid pressure. A gerotor pump pushes fluid whenever there is a differential speed, increasing fluid pressure inside a cylinder which pushes a piston that engages the clutches. Additional increase in differential speed also increases pumping action, which further increases fluid pressure, leading to increase in torque transfer or slip limitation. Shaffer further discloses a pressure-sensing valve that closes at a set pressure or fluid-flow velocity, thus, providing a sharp increase in slip limitation above a predetermined differential. However, the construction of this limited-slip differential requires a pump, valves, cylinder, piston, seals, and clutches that add to the complexity and cost of the device.
For the foregoing reasons, there is a need for a limited slip differential which limits the speed differentiation or xe2x80x98slipxe2x80x99 between the driven wheels and which gradually increases torque to the non-slipping wheel, as the rotational speed difference between the slipping wheel and the non-slipping wheel increases. The rate of increase of the torque to the non-slipping wheel should increase, as the speed difference between the slipping wheel and the non-slipping wheel increases. The rate of increase of the torque applied to the non-slipping wheel should preferably increase at a greater rate than the rate of increase of the speed difference between the driven wheels. Torque transfer should be substantially zero or minimal between both driven wheels, when there is substantially no speed difference or slip between the driven wheels, since such torque transfer is not required when there is substantially no speed difference between the driven wheels. The rate of increase of the torque applied to the non-slipping wheel should preferably increase at a greater rate than the rate of increase of the speed difference between the driven wheels. The rate of increase of the torque applied to the non-slipping wheel should be adjustable by modifying physical characteristics of the limited slip differential. Such limited slip differentials should be inexpensive, durable, long lasting, sturdy, easy to manufacture and install, either as an original equipment item or as a retrofit, be capable of having a substantially similar form factor and size as original equipment items, be easy to maintain, and require a minimum of maintenance. The limited slip differential should also be compatible with anti-lock braking systems, two, four, and multiple drive vehicles, and be useable in vehicle and other applications.
The limited-slip differential should provide substantial slip limitation or torque transfer substantially only when speed difference between the driven wheels is beyond normal vehicle driving limits. The torque transfer should increase as the differential speed continues to rise, which aids in decreasing the probability of a vehicle becoming immobilized during slippery conditions. The rate of increase of the torque applied to the non-slipping wheel should preferably increase at a greater rate than the rate of increase the speed difference between the wheels.
The present invention is directed to a limited slip differential that limits the speed differentiation or xe2x80x9cslipxe2x80x9d between the driven wheels, and gradually increases torque to the non-slipping wheel, as the rotational speed difference between the slipping wheel and the non-slipping wheel increases. The rate of increase of the torque to the non-slipping wheel should increase, as the speed difference between the slipping wheel and the non-slipping wheel increases. The rate of increase of the torque applied to the non-slipping wheel should preferably increase at a greater rate than the rate of increase of the speed difference between the wheels. Torque transfer should be substantially zero or minimal between both driven wheels, when there is substantially no speed difference or slip between the driven wheels, since such torque transfer is not required when there is substantially no speed difference or slip between the driven wheels. The rate of increase of the torque applied to the non-slipping wheel should be adjustable by modifying physical characteristics of the limited slip differential. Such limited slip differentials should be inexpensive, durable, long lasting, easy to manufacture and install, either as an original equipment item or as a retrofit, easy to maintain, and require a minimum of maintenance.
The limited slip differential has a sealed casing containing a fluid medium that houses opposing output gears and a plurality of opposing pinions having substantially parallel rotational axes. The opposing pinions mesh one with the other at the medial portions of the opposing pinions in a circumferential manner, and lateral portions of the opposing pinions mesh with opposing output gears, which are affixed to opposing driven shafts. Meshing between opposing pinions one with the other and meshing between opposing pinions and the opposing output gears respectively, function as gear pumps, which pump the fluid medium in the presence of relative rotation between the opposing output gears. Fluid pressure build-up caused by the pumping action results in pumping resistance, and, thus, slip limitation at the opposing output shafts of the limited slip differential, especially at high rotational speed differences between the opposing output shafts. Partitions substantially centrally located on each opposing pinions provide fluid barriers between the medial and lateral portions of each of the pinions, and, thus, promote pressure build-up of the pumped fluid medium.
The opposing output gears and the opposing pinions may have either helical teeth or spur teeth. Bladders may be optionally incorporated into the casing to accommodate fluid volume changes inside the casing, as a result of temperature variations. The number of meshing points that function as gear pumps, resulting from the continuous circumferential pinion arrangement, markedly increases the slip limiting capacity of the limited slip differential of the present invention, as compared with other limited slip differentials.
A limited slip differential having features of the present invention comprises a casing containing a fluid medium, said casing rotatably driven along an axis by an input member; plurality of opposing pinions therein said casing that are immersed in said casing fluid medium, having substantially parallel rotational axes, said opposing pinions meshing one with the other circumferentially at medial portions of said meshing opposing pinions; said pinions having partitions therebetween lateral and said medial portions of each of said opposing pinions; opposing driven gears, immersed in sail casing fluid medium, said opposing driven gears meshing with a plurality of said lateral portions; said opposing driven gears having output shafts adjoined thereto; said casing having opposing holes therethrough, said opposing output shafts rotatably mounted therethrough; and seals adapted to retain said fluid medium therein said casing.